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SubjectRe: r-o bind in nfsd
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> > > > I know there are a few cases, where filesystems call vfs_foo()
> > > > internally, where the vfsmount isn't available, but I think the proper
> > > > solution is just to fix those places, and not recurse back into the
> > > > VFS (which is AFAICS in all those cases totally unnecessary anyway).
> > > > This would make everybody happy, no?
> > >
> > > Apparmor can go play with itself. The proper fix is to lift the LSM nonsense
> > > into callers and leave vfs_...() alone;
> >
> > Maybe. I know precious little about this security thing, so I won't
> > argue about it's merits or faults. But:
> >
> > a) I have a hunch that the security guys wouldn't like to see the
> > order between permission() and security_foo() changed.
>
> It's their problem. Adjusting LSM methods, if needed, is up to LSM
> maintainers, whenever moving the hooks or code around those become
> convenient for kernel proper. According to Linus, IIRC.
>
> Especially since in this case they want to change prototypes anyway, so the
> churn is not an issue and having the hook called earlier is very unlikely to
> cause any problems.

CC-d linux-security-module, James Morris.

>
> > b) I fail to see how moving functionality to callers would improve
> > things
> >
> > > vfsmounts should *not* be passed there at all, with the exception of
> > > vfs_follow_link() which gets the full nameidata.
> >
> > Why?
>
> Because filesystem shouldn't _care_ where it is mounted. Anything
> vfsmount-dependent belongs to upper layers. The same goes for passing
> nameidata to fs methods, BTW - again, ->follow_link() is an obvious
> legitimate exception.

Nobody wants to send vfsmounts to the filesystem. But vfs_...() are
still part of the "upper layer", not the filesystem, so I'm not
convinced yet. For example:

-extern int vfs_mkdir(struct inode *, struct dentry *, int);
+extern int vfs_mkdir(const struct path *, struct dentry *, int);

There's one caller of vfs_mkdir that can't do this: cgroup_clone().
But that can call cgroup_mkdir() instead.

And having the vfsmount available within vfs_...() functions means,
that the mnt_want_write() check can be moved inside, which means that
callers get simpler and less likely to be buggy. Those are all
advantages IMO, regardless of any security module issues.

Miklos


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